Method of and apparatus for geophysical prospecting



Fhc13,1940. G. PQTAPENK@ m AL 190,323

METHOD OF AND APPARATUS FOR GEOPHYSCAL RGSPECTING Filed Jan. 4, 1938 2 Shader-Sheet I,

ATTORNEY.

Feb. 13, i940. s. Po'rAPENK r-.T AL n 2,190,323

METHOD 0F AND APPARATUS FOR GEOPHYSICAL PROSPECTING Filed Jan. 4. 1938 2 Sheets-Sheet 2 Mil- 27 /A/PUT ATTORNEY.

Patented Feb. 13, 1940 ,METHOD F AND APPARATUS FOB GEOPHYSICAL PBOSPECTING Gennady Potapenko and Donald Folland, Pasa-v dena, Calif., assignors to Geo-Frequenta Corporation. a corporation o! Delaware Application January 4. '1938, Serial No. 183,264V

9 Claims.

Our invention relates to geophysical prospecting by determining the electrolytic polarization characteristics ofthe underlying earth. and to electrical switching apparatus employed in mak- ,5 ing such determinations.

An object of our invention is to provide, a meth-v od of measuring' the electrolytic polarization characteristics of the earth.

Another object is to provide apparatus for 10 continuously measuring earth polarization.

Another object is to provide automatic switching apparatus for repeating a sequence of circuit connectionsv with predetermined intervening time intervals.

l These and other' apparent objects we attain in a manner which will be clear from a consideration of the following description taken in connection lwith the accompanying drawings, oi which,

Fig. 1 is an illustration of an apparatus employed to demonstrate the phenomenon of electrolytic polarization.

Fig. 2 shows the manner in which the current in the circuit of Fig. 1 Varies with time. Fig. 3 shows the manner vin which the `electromotive force of polarization varies with time in the circuit of Fig. 1.

Fig. 4 is a diagrammatic illustration of an apparatus employed in the practice of our invention.

Fig. 5 shows the sequence of events occurring rent starts flowing in a quantity determined by Ohm's law and represented by In in -li'ig. 2. In

ordinary electrolytes, however, the current I immediately starts to decrease as shown in Fig. 2 and eventually reaches afminimum value Imm.

at time T1. This decrease in current isduetoV polarization at inhomogeneities in the electrolyte,

which inthe -iampiiu'atus of Fig. l exist at the surfaces of the electrodes. Negative' charges concentrate f near the surface of electrode l! and positive 'charges concentrate near the surface o! electrode |3, and these chargesin eilect result in a counter-electromotive force inthe electrolytic circuit, which is the electromotive force o! polarization. This E. M. F. or polarization, iollowing the closing of switch i6 at time To builds up as shown in Fig. 3 to a maximum value emu. 5 at ltime T1 and, if the switch I6 is opened at time T1, decreases, as shown, substantially to zero at time T2. 'I'he E. M. F. oi polarization during the discharge period Ti Tn is evidenced by a potential difference between the electrodes, 10 which is proportional to the E. M. F. of polarization, and by continuouslymeasuring this potential diierence one may determine accuratelythe manner in which the E. M. F. of polarization discharges. When oil is employed as the liquid 15 i I, similar phenomena occur `except that the de'- crease in current is not observed to occur until after a delay of about 1 second, While for elec-v trolytes only partially consisting of oil the polarization' characteristics lie intermediate those for 20 pure oil and those for electrolyte without oil.

Since the earth conducts electricity mainly as an electrolyte, polarization may be observed in the earth, as in the apparatus oi Fig. 1. 'I'hev earth is composed largely of strata and lenticular 25 deposits of many diiIerent substances and provides many inhomogeneities at which polarization charges may concentrate, and since the various substances comprising the earth exhibit dif- -ferent .polarization characteristics, the proper 30 measurement of these characteristics may serve as a very eiilcient tool in geophysical prospectlng to locate and identify the formations and structure o1 underlying earth.

In the practice o! our invention we may employ the apparatus of Fig. 4. Electrodes 2l and 2| are electrically connected to the earth 22 and are adapted to be connected for a limited time period in a polarizing circuit with the source 2l o o! unidirectional current. Electrodes 24 and 2|, preferably ci the non-polarizing type, are also electrically connected to the earth and are adaptved to be connected at-suitable times to a measuring circuit comprising a direct current am- 45' pliiier It; the output ot which is connected toa cathode ray oseillograph 21 in such manner that the oscillograph continuously measuresV and records the potential diilerence between the measuring electrodes 2l and 2l during the dle- 50 charge oi earth polarization -iollowing the polarislng period. -Since the potential dinerence between the electrodes 2l and 2l during discharge o! polarlmtion is proportional to the E. H. F. of polarisation then existing. this method provides 55 an accurate way of determining the manner of variation of earth polarization.v

The apparatus employed is preferably adapted to complete the polarizing circuit at time t1 (Fig. 5) and to open the polarizing circuit at time t2. In this polarizing period, an E. M. F. E is applied to the electrodes and 2| and the E. M. F. of polarization of the earth is building up. At time t2 the applied E. M. F. E drops to zero and the E. M. F. of polarization gradually decreases as shown in Fig. 3 in the period T1 T2. At time tz the electrodes 24 and 25 are connected to the amplifier 26 andthe cathode ray oscillograph 21 is connected for operation. At time t3 the measuring circuit between electrodes 24 and 25 is opened and the operation of the oscillograph 21 is interrupted. During the time interval t2 t3 the potential .difference between electrodes 21| and 25 is continuously measured by the oscillograph and a record similar to curve V is made by the 'oscillograph At time t4 the polarizing circuit is preferably again connected and the entire sequence of events is againrepeated. The apparatus is preferably adapted to repeat this sequence indenitely, and by this means an average curve under equilibrium conditions is obtained which is better than that obtainable from a single cycle such as between ti and t4. v-

The time period t1 t2 during which the polarizing E. M. F. is applied and the time period t2 t3 during which the polarization is discharging and is measured by the oscillograph are always greater than 1 second and are generally made of the order of 10 to'20 seconds duration. By this procedure we eliminate all effect of transient phenomena which take place in relatively short time intervals following application of an applied E. M. F., and which have nothing to do with the polarization phenomena with which we are concerned, but which are rather dependent upon the electrical constants of .the connecting circuits as well as of the earth circuit.

In the apparatus of Fig. 4, we employ a fourpole, double-throw relay 30, a five-pole, doublethrow relay 3|, and a three-pole, double-throw relay 32 whose operation is controlled by the thyratron tubes 33, 34, and 35, respectively. The cathodes of all the thyratrons are connected togetherand to the positive terminal of the direct current source 36. Heaters for the cathodes of the thyratrons are suitably energized from an electrical source. The heaters are eliminated from the drawings in the interest of clearness. Connected in parallel across the source 35 are the potentiometers 31,4 33, and 33, the movable arms of which are connected to the grids of the thyratrons 33, 34, and 35, respectively. A source of direct currenthas its negative terminal connected to the cathodes of all the thyratrons and its positive terminal connected to the switch arm of switch 30a of relay 30. The upper contact of switch 30a is connected through the variable resistance 4| and the operating coil 42 of relay 30 to the anode of thyratron 33; and between the cathodes ofthe thyratrons and a point between resistance 4| and coil 42. is connected a condenser 43 of suitable capacity. In a similar manner the upper contacts of the switches 3|a and` 32a are connectedrespectively tothe variable resistances 44 andv 45, ywhich connect through the operating coils 46 and 41, respectively, of the relays 3| and 32, respectively, to the anodes of thyratrons 34 and 35, respectively. The condensers 48 and 43 are connected in the circuits of thyratrons 34 and 35, respectively, in a manner similar to that in which condenser 43 is connected in the circuit of thyratron 33. The normal positions ofthe switch arms of the three relays is up, contacting the upper contacts-and the energization of the operating coils 42, 46, and 41 causes the relays which they control to operate and the switch arms thereof to be contacted to the lower contacts of the relays until reset to their normal positions as will hereinafter be explained.

The operation of each of the thyratron circuits in controlling its associated relay may be understoodfrom consideration of the operation o'f the control circuit of thyratron 33. With thel arms to contact the lower contacts of the relay.Y

The time between resetting of the relay 30 to contact its upper contacts and operation of the relay as described above is controlled by the product of the resistance 4| and the capacity of condenser 43, the time being greater for greater values ofl this product. By variation, then, of the resistance 4|, the time elapsing before operation of relay 30 may be varied.v

The lower contact of switch 30a is connected to the switch arm of switch 3|a, and the lower contact of switch 3|a is connected to the switch arm of switch 32a. It will be clear, then, that operation of relay 30 energizes the control circuit of thyratron 34 which after a time interval, determined by the product of resistance 44 and the capacity of condenser 48, operates in the man- -operation of its associated relay 32 after a time interval determined bythe resistance and the capacity` of condenser 45.

According to our invention, the operation of the relay 32 is adapted to cause resetting of all the relays and in this manner initiate a repetition of the entire sequence of switching events. This is accomplished by providing the relays 30, 3|, and 32 with reset coils 50, 5|, and 52, respectively, which when energized are adapted to reset their associated relays in normal position with their switch yarms contacting their upper contacts. The reset coil 50 is connected between 'one terminal of an electrical source 53 and the lower contact of switch 30d. The reset coil 5| is connected between the same terminal of source 53 and the lower contact of switch 3|e. The reset coil 52 is connected, through a resistor 54, between the same terminal of source 53 and the switch arm ofl switch, 32e. The other terminal of source 53 is connected through switch with the lower contact of switch 32e. The switch arms of switches 30d, 3|e, and 32e are all connected together. When relay 32 operates so as to contact switch 32e to its lower contact, all of the reset coils 50, 5|, and 52 are energized and the relays 30, 3|, and 32 are reset to their normal positions with the switch arms up, whereupon the sequence of switching events again repeated. The resistor 54 assures slower action of the reset coil 52 and causes the relay 32 to be the last to reset. Should serves as a source of electrons.

it be desired to stop the operation of the switching apparatus at the end of any one cycle, it is only necessary to-open switch 66.

The polarizing and measuring circuits and their connections to the relay switches will now be described. The electrode 260i the polarizing circuit is connected to the lower contact of switch 36h. 'I'he switch arm of switch 66h is connected to the switch arm of switch 6Ib. 'Ihe upper contact of switch lib is connected to one terminal of the source 26, the other terminal of which is connected to electrode 2|` of the polarizing circuit. It will be clear that operation of relay 36 closes the polarizing circuit, while operationof relay 3| opens the polarizing circuit. The electrode 24 of the measuring circuit is connected to one of the input terminals of the direct current. amplifier 26, the other input terminal of which-is connected to the electrode 26 ofthe measuring circuit through switch 3Ic when contacting its lower contact. Thus operation of relay 3l connects the measuring electrodes to the amplier 26 so that the potential difference between the electrodes may be measured. The direct current amplifier 26 is adapted to amplify the relatively small potential difference between the electrodes 24 and 26 to a value suitable to inuence in asatisfactorily measurable degree the oscillograph 21. Such ampliers are well known in the radio art, a representative circuit being such as that shown in Electrical Engineers Handbook, 3rd edition, John lWiley 8: Sons. vol. 5, section 14, page 7.

While oscillographs of various other types may be used, we prefer to employa cathode ray oscillograph to continuously measure the output from the ampli-lier 26 during the chosen measuring period. Such a cathode ray oscillograph comprises an' evacuated envelope `66 having exposed to view a ucrescent screen 6| which is sensitive to the impact of electrons thereon. A illament 62 is heated by current from electrical source 63 and The electrons are accelerated by an electric eld through a potential difference created by battery 64, and are directed in a well-defined beam of small cross-section toward the screen 6I. In passing between the plates 66 and 66, which are dis- Y posed vertically one above the other, the moving electrons are diverted in a vertical direction, either up or down, depending upon the relative electr'ic potentials of the plates 66 and 66. By

impressing on the plates 66 and 66 the varying potential diierence which it is desired to observe, the light spot on the screen 6I is caused to move up and down in conformity with the varying potential difference. The platesI 66 and 66 are connected to the output terminals of the amplier 26, and are therefore, subjected to a potential difference proportional to that Vbetween electrodes 24 and 26. In order to cause the spot oi' light on screen 6| to trace aecurve representing the variation of themeasured potential difference withtime, the electron beam is caused to pass between the plates 61 and 66 which are disposed horizontally with respect to one another, and a potential difierence varying uniformly withtime is applied to the plates 61 andy 6B. The vertical movement of the light spot is thus accompanied by a uniform horizontal move- Y, ment.

The uniformly varying potential diner- Y .ence is'provided b'y a condenser in the process of being charged. To this end, the plate 66 is connected to vone terminal of each of condensers, .69 and 16, which are of dinerentvalus; and

double-throw switch 1I, which in one position connects to the other terminal `of condenser 66 and in the other position connects to the other terminal of condenser 16. The movable arm of switch 1| is also connected through variable resistance 'I2 to the switch arm of switch Sid. The plate 66 is connected to one terminal ofthe battery 16, the other terminal of which is connected to the upper contact o1' the switch 62h. The lower contact of switch 3Id is connected to the switch arm of switch 62h, completing a charging circuit, spoken of as the sweep circuit comprising battery 16, resistance 12 and one of condensers 66 and 16 when the relay 6| is in' which time period may be varied by changing thc resistance 4|. At time ti relay 66 operates andapplies polarizing electromotive force E to the electrodes 26 and 2| for a time period t1 t: which may be varied by changing resistance 44. At time t2 the relay 3| operates, opening the polarizing circuit, connecting the measuring electrodes 24 and 26 to the ampliiler 26; and energizing the sweep circuit., The oscillograph is connected for measuring the potential dierence 'between electrodes 24 and 26 during the time period t: ta, the duration. of which may be varied by changing resistance 46. In this period the potential difference between the measuring electrodes may vary with timeaccording to the curve V and be shown on the `oscillograph as a curve with this form. Assuming condenser 69 to be connected in circuit by switch 1|, charging of condenser '66 by battery 13 through resistance 12 is started at time tz, the potential diierence across the condenser building up as dotted line S. The rate of charge may be adjusted by varying resistance 12 or by shifting switch 1| to` connect condenser 16 of different capacity. As

the potential diiference S across the condenser, and therefore across the plates 61 and 66, increases, the light spot on the oscillograph screen travels horizontally as is necessary to assure tracing of the curve V. At time t: the relay 62 operates disconnecting the sweep circuit and resetting all relays'totheir normal positions with thc'switch arms contacting the upper contacts. At the resetting of relay 66 the condenser 66, or condenser 16 if it is in circuit, is shorted by switch 66c,causing the potential diiierence S to fall to zero where it remains until charging again starts at time ts. Operation ci' relay 66 then takes place at time t4 and the entire sequence of operations is repeated indennitely.

The curve V traced by the spot of lighten the oscillograph screen may be photographed to provide a record oi' the variation with time of electrodes 24 and26, or the movement of the spot of light may be visually observed and traced by means oi pencil and paper. For visual lobservation particularly, we prefer to use an oscillograph tube having a screen containing phosphorescent or other suitable material causing light to continue to come from each position o cupied by the light spot for a considerable afterthespothasmoved'on. Bythismeans the ventire curve V traced by the light spot remains constantly visible on the screen.

The time intervals composing ,the cycle of events described may be varied tomeasure the change in polarization under diierent conditions, but we preferably make the interval t2 t@ suiiiciently long to permit the earth polarization to reduce substantially to zero. We may make the interval t1 t2 suiiiciently long to assure a maximum polarization as in Fig. '3, or we may make this time interval of various values less than that required to reach the maximum and observe the discharge curve V under each condition. We may also conduct each or all of these measurements with various values of the applied potential' difference E. ln any case, the periods t1 .tz and t2 t3 are made longer than l second, as previously described.

In conducting eld operations employing the method and apparatus herein disclosed, we preferably make any or all of the above-described measurements at each of a plurality of electrode spacings. The greater the electrode spacing, the greater is the depth of `formation which can aiest the measured polarization; so by varying the electrode spacing and noting at what separation. of electrodes a certain polarization eiect becomes noticeable, it is possible to estimate the approximate depth of the formation responsible for it.

We may vary the spacing of both pairs of electrodes together, or we may move the electrodes 24 and 25 around to explorey the surrounding region for each setting of the electrodes 20 and 2 I.

It is understood that variations andfmodincations in the method and apparatus disclosed herein may be made by those skilled in the art without departing from the spirit of the invention defined by the appended claims.

We claim as our invention:

1. A method of geophysical prospecting comprising polarizing the earth for a limited time period by application thereto of a unidirectional electromotive force, and continuously measuring the instantaneous value of the resultant earth polarization during discharge of said polarization following said polarizing period.

2. A method of geophysical prospecting comprising polarizing the earth for a limited time period by application thereto of a unidirectional electromotive force, and continuously measuring between spaced points in the earth the instantaneous value of the potential dierence due to A earth polarization during discharge of said polarization following said polarizing period.

3. A method of geophysical prospecting comprising subjecting the earth to intermittent polarizing applicationsof a unidirectional electromotive force, and continuously measuring the instantaneous value of the resultant earth polarizationduring lportions of the discharge intervals between said polarizing applications.

4. A method of geophysical prospecting comprising subjecting the earth to intermittent polarizing applications of a unidirectional electromotive force, and continuously measuring between spaced -points in the earth the instantaneous value of the potential difference due to earth polarization during discharge of said polarization inthe intervals between said polarizing applications.

5. In combination, a pair of spaced electrodes electrically connected to the earth, a source of .unidirectional electr-emotive force connectedin a polarizing circuit with said electrodes, a lsecond pair of spaced electrodes electrically connected to amores the earth, a cathode ray oscillograph connected in a measuring circuit and responsive to the pol tential diierence between said second pair of electrodes, a sweep circuit for said oscillograph, and switching means adapted to intermittently close said polarizing circuit during polarizing periods, and close said measuring circuit and said sweep circuit during discharge of the resultant earth polarization in the intervals between said polarizing periods.

6. In combination, a pair of spaced electrodes electrically connected to the earth, a source of unidirectional electromotive force connected in a polarizing circuit with said electrodes, a second pair of spaced electrodes electrically connected to the earth, a cathode ray oscillograph connected in a'measuring circuit and responsive to the potential diierence between said second pair of electrodes, a sweep circuit for said oscillograph including a condenser to be charged, three relays each providing switches adapted to occupy normal and operated positions, three vacuum tubes each controlling the operation of one of said relays and causing operation of its associated relay at a time following its energization by a predetermined time interval, connections between said relays for permitting operation of each 'relay to energize the vacuum tube 'associated with the succeeding relay, and means for resetting the switches of all of said relays to normal position and controlled by operation of the last of said relays to operate, switches of one of said relays being connected upon operation to close said polarizing circuit and to short the condenser of said sweep circuit, switches of the succeedingy relay being connected upon operation to open said polarizing circuit, close said measuring circuit.

` and energize said sweep circuit, and switches of the third of said relays vbeing connected upon operation to de-energize said sweep circuit, whereby said polarizing circuit is intermittently closed during polarizing periods and said measuring and sweep circuits are closed during discharge` of the resultant earth polarization in the intervals between said polarizing periods.

7. Switching apparatus comprising in combination, a plurality of relays each providing switches adapted to occupy normal and operated positions, a plurality of vacuum tubes each controlling the operation of one of said relays and causing operation of its associated relay at a time following its energization by a predetervmined time interval, connections between said relays for permitting operation-of each relay to energize the vacuum tube associated with the succeeding relay, and means for resetting the switches of all of said relays to normal position and controlled by operation of the last of said relays to operate. l y

8. Switching apparatus comprising in combination, a plurality of relays each providing switchesadapted to occupy normal and operated positions, a plurality of vacuum tubes each controlling the operation of one of said relays and causing operation of its associated relay at a time following its energlzation by a predetermined time interval, connections between said 'relays for permitting operation of each relay to energize the vacuum tube associated with the succeeding relay, a reset coil associated with to reset the switches of its associated relay to normal position, and means adapted to energize all of said reset coils upon operation of the last of said relays to operate.

` each of said relays and adapted when energized 9. Switching apparatus comprising in com- '-bination, a'plurality of relays each providing -through a control circuit, each of said control circuits comprising an electrical resistance and the operating `coil of the associated relay in series with the anode of the associated thyratron and,

a condenser connected between the cathode of said thyratron and a point between said resistance and said operating coil whereby said thyratron breaks down and allows current to pass through said operating coil to operate said relay at a 4u predetermined time following energization of'said control circuit, a source ol.' direct current adapted to energize the control circuit of the thyratron associated with the ilrst of said relays through a switch of said first relay when in its normal position, connections between said relays for permitting operation of each relay to energize the control circuit of the thyratron associated with the succeeding relay, a reset coil associated with each of said relays and adapted when energized to yreset* the switches of its associated relay to normal position, and means adapted to energize all of'sald reset-coils upon operation of the last of said vrelays to operate.

\ n l GENNADY Po'rAPENKo.

/ DONALD mLLAND. 

